1. Field of the Invention
The present invention relates to hydrogen-absorbing alloys used for, for example, negative electrode active materials in negative electrodes of nickel-metal hydride storage batteries and heat pumps, and a nickel-metal hydride storage battery employing such a hydrogen-absorbing alloy for the negative electrode. More particularly, the invention provides a hydrogen-absorbing alloy that can be effectively used for, for example, a heat pump or a negative electrode active material in the negative electrode of a nickel-metal hydride storage battery.
2. Description of Related Art
Conventionally, hydrogen-absorbing alloys have been used in various fields, such as heat pumps and negative electrode active materials in negative electrodes of nickel-metal hydride storage batteries.
The hydrogen absorbing alloys commonly used for the negative electrode active materials of nickel-metal hydride storage batteries include rare earth-Ni hydrogen absorbing alloys having a CaCu5 crystal structure as its main phase, and AB2 Laves phase hydrogen absorbing alloys containing Ti, Zr, V, and Ni.
In recent years, the nickel-metal hydride storage batteries have been used in various applications such as various portable devices and hybrid electric automobiles. As a result, the demand for higher capacity in the nickel-metal hydride storage batteries has been increasing, and likewise, the demand for higher performance in heat pumps has been rising.
However, since the hydrogen absorbing alloys mentioned above do not have sufficient hydrogen-absorbing capability, it has been difficult to achieve sufficient capacity in nickel-metal hydride storage batteries, and it has also difficult to achieve higher performance in heat pumps.
In recent years, it has been found that a hydrogen absorbing alloy having a Ce2Ni7 type or a CeNi3 type crystal structure, rather than the CaCu5 type, obtained by adding Mg or the like to a rare earth-Ni hydrogen absorbing alloy having the CaCu5 crystal structure as its main phase, achieves higher hydrogen-absorbing capability than the conventional rare earth-Ni hydrogen absorbing alloys. For example, Japanese Published Unexamined Patent Application 11-162459A proposes the use of such a rare earth-Mg—Ni hydrogen absorbing alloy for, for example, the negative electrode active material in the negative electrode of a nickel-metal hydride storage battery.
The just-described rare earth-Mg—Ni hydrogen absorbing alloy, however, shows a larger hysteresis between the pressure changes during the hydrogen absorption and during the hydrogen desorption than the rare earth-nickel hydrogen absorbing alloy having a CaCu5 crystal structure as its main phase. The pressure during desorption of hydrogen becomes lower than the pressure during absorption of hydrogen, and also the hydrogen absorbing alloy tends to deteriorate easily. Consequently, the problems arise that, when the rare earth-Mg—Ni hydrogen absorbing alloy is used for the negative electrode of a nickel-metal hydride storage battery, the battery discharge voltage lowers, and the hydrogen absorbing alloy deteriorates due to the charge-discharge operations, resulting in deterioration in the battery cycle performance.
In addition, when the just-mentioned rare earth-Mg—Ni hydrogen absorbing alloy is used for a heat pump, the problems arise that the use temperature range is limited since the cooling and heating capability of the heat pump is low, and that the efficiency of the heat pump becomes poor since the amount of available heat is small.
In recent years, it has been proposed to limit the amount of Ce relative to that of La in the rare earth-Mg—Ni hydrogen absorbing alloy to less than a predetermined amount to enhance the degree of uniformity of Mg concentration in the hydrogen absorbing alloy so that the difference in equilibrium pressure between the hydrogen absorption process and the hydrogen desorption process can be reduced. This helps to prevent the discharge voltage from lowering when the rare earth-Mg—Ni hydrogen absorbing alloy is used for the negative electrode of a nickel-metal hydride storage battery. (See, for example, Japanese Published Unexamined Patent Application 2000-265229.)
Nevertheless, even when the amount of Ce relative to La in the rare-earth-Mg—Ni hydrogen absorbing alloy is limited to less than a predetermined amount to enhance the degree of uniformity of Mg concentration in the hydrogen absorbing alloy, it has still been difficult to sufficiently reduce the difference in equilibrium pressure between the hydrogen absorption process and the hydrogen desorption process.